1. Field of the Invention
The present invention relates to a planar light source device and to a liquid-crystal display device comprising it, More precisely, the invention relates to a technique for reducing the weight of a planar light source device, and to a liquid-crystal display device that comprises the planar light source device favorably as the backlight system therein.
2. Description of the Related Art
Transmission-type liquid-crystal display devices are much used these days as monitors for personal computers and as display devices for thin TVs, and in such liquid-crystal display devices, in general, a planar lighting device, or that is, a backlight is disposed on the back of the liquid-crystal element therein.
The backlight has the function of converting the linear light from a linear light source such as a cold-cathode tube into planar light, Typical examples of the concrete structure of backlight are mentioned. One comprises a light source disposed just below the back of a liquid-crystal element; and the other comprises a light source disposed on the side thereof in which the light from the light source is converted into planar light though a transparent light guide such as an acrylic plate to obtain a planar light source (sidelight system). An optical element such as prism array or diffusion sheet is disposed on the light output side of the device, and a light reflector formed of a foamed polyester or the like is disposed oh the other side opposite to the light output side thereof to thereby obtain the desired optical characteristics.
The sidelight system is disclosed in, for example, JP-A 61-99187 and 63-62014. In particular, in order to more effectively utilize the general properties of lightweight and thin liquid-crystal display devices, it is desirable to use the sidelight system in which the backlight may be thin. Accordingly, sidelight-system backlights are much used in liquid-crystal display devices such as portable personal computers.
The necessary properties of such backlights are being to be on a higher level these days. In particular, in monitor display devices for notebook-side personal computers and desk-top personal computers and in large-panel thin TVs, generally used are transmission-type full-color liquid-crystal display devices. In this case, since the light transmittance of the color liquid-crystal cell is extremely low by itself, the necessary brightness of the backlight source must be inevitably high.
Accordingly, in the above-mentioned sidelight-system backlight, generally but much used is a sheet of prism array or the like so as to ensure the front brightness of the device owing to the optical light-condensing effect thereof, or a special photofunctional sheet having a light-deflecting and converting function so as to effectively utilize the output light of the device. However, much using it inevitably results in the increase in the weight of the planar light source device and therefore the increase in the weight of the liquid-crystal display device comprising it.
For solving the problem, it may be taken into consideration to use a resin that is more lightweight than ordinary acrylic resin for light guides.
However, when the light guide formed of a lightweight material is contacted with an optical reflector, then there occurs a problem in that the light guide is scratched and could not be in practical use.